The Bush Administration set a stricter standard in 2008, but critics, including Oregon air regulators, said it didn't go far enough.

The new "primary" standard would reduce allowable levels of ground-level ozone, the main ingredient of smog, by up to 20 percent, from 75 parts per billion of ozone over an eight-hour period to between 60 to 70 parts per billion, the EPA said Thursday.

If the EPA opts for the tightest limit, 60 ppb, in a final rule to be issued by August, all four areas regularly monitored in Oregon -- Portland, Salem, Eugene and Medford -- would be out of compliance based on recent monitoring results. In 2009, the four areas all met the least stringent limit in the EPA's range, 70 ppb, though they have exceeded it in prior years.

Ground-level ozone is the main ingredient of smog. It results when car exhaust, chemical solvents, power plant pollution and industrial emissions react to sunlight. It's worst on hot days with stagnant air.

In 2008, independent scientific panels advising EPA unanimously recommended the tighter range EPA advocated Thursday, citing new health research and strong concerns about smog's effects on hundreds of thousands of children with asthma.

But the agency said then that the new health research was too uncertain. Industry and utilities also opposed tighter standards, which could hit coal plants particularly hard.

Oregon regulators asked the EPA in 2008 to go with the tougher limits, noting that Oregon's asthma rates are well above the national average, and had been considering whether to set tighter state standards for ozone before the EPA's Thursday's announcement.

Andy Ginsburg, air quality administrator for the Oregon Department of Environmental Quality, said the new standard will address the state's concerns, ensuring that meeting it truly means an area's residents aren't suffering significant health damage from smog.

If the EPA opts for the tightest level in the range, 60 parts per billion, "it would be a big challenge for us, but it would be achievable," Ginsburg said.

Other areas of Oregon could come into play: Hermiston and Bend, both sampled recently, had ozone levels at or above the 60 ppb threshold, DEQ said. Proposed tightening of a separate "secondary" standard aimed at cutting ozone's damage to crops and plants could also push some rural areas out of compliance, Ginsburg said.

The federal government can withhold highway money if states don't comply. But the more likely consequence is that Oregon would expand vehicle emissions testing, require more pollution control at factories and power plants and expand retrofits of diesel engines to reduce emissions of nitrogen oxides, a key component of ozone.

At 60 ppb, the new standard would yield health benefits from $35 billion to $100 billion a year by 2020, the EPA estimated, eliminating 58,000 cases of aggravated asthma and from 4,000 to 12,000 premature deaths that year.

Annual costs would run from $52 billion to $90 billion by 2020, the agency said.

EPA plans to finalize new ozone standards by August, with states required to meet the new primary standard by 2014. For details: http://tinyurl.com/epaozone.